In a wireless communication system, for providing a variety of broadband information services, there is a constant demand for increasing the transmission rate. It is possible to increase the transmission rate by widening the communication bandwidth. However, since the available frequency band is limited, an improvement in the frequency efficiency is necessary. As a technology for significantly improving the frequency efficiency, attention is being focused on a Multiple Input Multiple Output (MIMO) technology for performing wireless transmission by using a plurality of transmit-and-receive antennas, and MIMO technology has been put to a practical use, for example, in a cellular system and a wireless LAN system. The degree by which the frequency efficiency is improved by using MIMO technology is proportional to the number of transmit-and-receive antennas. However, the number of receive antennas that can be disposed in a terminal device (wireless receiving apparatus) is limited. Thus, the following multi-user MIMO (MU-MIMO) technology is effective for improving the frequency efficiency. In MU-MIMO, a plurality of terminal devices connected at the same time are regarded as a large-scale virtual antenna array, and a signal transmitted from a base station device (wireless transmitting apparatus) to each terminal device is spatially multiplexed.
In MU-MIMO, signals transmitted to individual terminal devices are received by the terminal devices as inter-user interference (IUI). Accordingly, it is necessary to suppress IUI. For example, in 3.9 G mobile wireless communication system (called LTE) Release 8 (Rel. 8), the following linear precoding is employed. In linear precoding, a base station device multiplies a signal in advance by a linear filter calculated on the basis of channel state information supplied from each terminal device, thereby suppressing IUI. However, unless the orthogonality of channels between terminal devices which receive spatially multiplexed signals is high, it is not possible to effectively suppress IUI. Thus, in MU-MIMO based on linear precoding, there is a limitation on improving the frequency efficiency.
These days, attention is being focused on a MU-MIMO technology using nonlinear precoding in which nonlinear processing is performed in a base station device. If a terminal device is capable of performing modulo calculations, a perturbation vector using, as an element, a complex number (perturbation term) obtained by multiplying a certain Gaussian integer by a constant real number may be added to a transmitting signal. Thus, in accordance with the state of channels between a base station device and a plurality of terminal devices, appropriate perturbation vectors are set. Then, even if the orthogonality of channels between terminal devices which receive spatial multiplexed signals is not high, it is possible to considerably reduce required transmission power compared with the use of linear precoding in which perturbation vectors are not added.
The transmission performance of nonlinear precoding considerably varies depending on a search method for perturbation vectors. For example, Vector Perturbation (VP) disclosed NPL 1 is a technique for searching for the optimum perturbation vector from all selectable perturbation vectors. VP implements excellent transmission performance, but on the other hand, an enormous amount of calculations is required. On the other hand, by using a method based on Tomlinson Harashima Precoding (THP) disclosed in NPL 2, a perturbation vector can be simply searched, but transmission performance is considerably decreased compared with the use of VP.
Nonlinear precoding is an effective technique for improving the frequency efficiency. On the other hand, however, in wireless transmission in which modulo calculations, which are required in nonlinear precoding, are performed, a factor in decreasing the performance, which is called modulo loss, is involved. Accordingly, depending on the state of channels, there may be a case in which modulo loss surpasses the effect of reducing required transmission power obtained by nonlinear precoding. In such a case, the transmission performance may be decreased compared with the use of linear precoding. Accordingly, for example, NPL 3 discloses the following method. Processing similar to linear precoding, which does not require modulo calculations, is performed on a signal to be transmitted to a terminal device in which there is only a small effect of reducing transmission power obtained by modulo calculations. In contrast, nonlinear precoding based on THP using modulo calculations is performed only on a signal to be transmitted to a terminal device in which the level of IUI is high, and thus, a great effect of suppressing transmission power by performing modulo calculations can be expected.
The application of nonlinear precoding is not restricted to MU-MIMO. For example, in NPL 4, nonlinear precoding is utilized in order to reduce the peak to average power ratio (PAPR), which causes a problem in orthogonal frequency division multiplex (OFDM) transmission. In this case, too, a determination as to whether nonlinear precoding will be performed is made depending on actually measured PAPR and modulo loss produced in a wireless receiving apparatus.
Nonlinear precoding is performed, provided that modulo calculations are performed in a terminal device. However, since nonlinear precoding involves a factor in decreasing the performance, which is called modulo loss, there may be a case in which linear precoding and nonlinear precoding are adaptively switched in accordance with, for example, the state of channels, as in NPL 3. It is thus necessary for a terminal device to know whether or not modulo calculations will be required for a received signal or a perturbation term is added to a transmitted signal. Accordingly, NPL 5 discusses a technique for supplying control information which enables a terminal device to know whether modulo calculations will be required. However, supplying control information increases the overhead, thereby imposing a restriction on an improvement in the frequency efficiency.